Question 19M.3.SL.TZ1.8a
| Date | May 2019 | Marks available | [Maximum mark: 1] | Reference code | 19M.3.SL.TZ1.8a |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Estimate | Question number | a | Adapted from | N/A |
Hexochlorophene (HCP) and ortho-phenylphenol (OPP) are two ingredients of handwashing formulations. The antiseptic effectiveness of the two agents was tested by soaking discs in the agents and applying them to plates inoculated with Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. The resulting zones of inhibition are depicted.
[Source: © International Baccalaureate Organization 2019]
Estimate the diameter of the zone of inhibition around the disc containing OPP in the S. aureus culture.
[1]
14–16 mm or 1.5–3 mm «around the disc» ✔
Units required
The zone of inhibition was surprisingly poorly measured, even given the range and alternative measurements in the mark scheme. It is possible that when asked to estimate, candidates did not use a ruler.
